With progress in the optical communications based on a dense wavelength division multiplexing transmission system over the recent years, a higher output is increasingly demanded to a pumping light source used for the optical amplifier.
Further, a greater expectation is recently given to a Raman amplifier as an amplifier for amplifying the light having a much broader band than by an erbium-doped optical amplifier that has hitherto been employed as the optical amplifier. The Raman amplifier may be defined as a method of amplifying the optical signals, which utilizes such a phenomenon that a gain occurs on the side of frequencies as low as 13 THz on the basis of a pumping wavelength due to the stimulated Raman scattering occurred when the pumping beams enter an optical fiber, and, when the signal beams having the wavelength range containing the gain described above are inputted to the optical fiber in the thus excited state, these signals are amplified.
According to the Raman amplification, the signal beams are amplified in a state where a polarization direction of the signal beams is coincident with a polarization direction of the pumping beams, and it is therefore required that an influence by a deviation between polarization directions of the signal beams and of the pumping beams be minimized. For attaining this, a degree of polarization (DOP) has hitherto been reduced by obviating the polarization of the pumping beams, which may be called depolarization.
As a method for depolarizing a laser beam emitted from a conventional semiconductor laser module used as a pumping light source or so in the optical fiber amplifier, one in which two laser beams are polarization-synthesized and output from an optical fiber is known.
FIG. 21 is an explanatory diagram showing a conventional semiconductor laser apparatus as disclosed in U.S. Pat. No. 5,589,684.
As shown in FIG. 21, the conventional semiconductor laser apparatus comprises a first semiconductor laser device 100 and a second semiconductor laser device 101 each emitting a laser beam of the same wavelength in a direction vertical to the other; a first collimating lens 102 configured to collimate the laser beam emitted from the first semiconductor laser device 100; a second collimating lens 103 configured to collimate the laser beam emitted from the second semiconductor laser device 101; a polarization-synthesizing coupler 104 configured to polarization-synthesize the orthogonally polarized laser beams that were collimated by the first collimating lens 102 and the second collimating lens 103; a convergent lens 105 configured to converge the laser beams polarization-synthesized by the polarization-synthesizing coupler 104; and an optical fiber 106 for receiving the laser beams converged by the convergent lens 105 and letting the laser beams travel outside.
In the conventional semiconductor laser apparatus, the laser beams are emitted from the first semiconductor laser device 100 and the second semiconductor laser device 101 in mutually vertical directions and are polarization-synthesized by the polarization-synthesizing coupler 104 to obtain a laser beam of reduced DOP from the optical fiber 106. (This technology will hereinafter be called a prior art 1.) In addition, Japanese Patent Application Laid-open No. Sho 60-76707 discloses a semiconductor laser module including a first and a second semiconductor laser devices disposed on a heat sink and emitting a first and a second laser beam respectively with mutually parallel optical axes and mutually parallel polarization directions from a substantially identical light-emitting end faces; a polarization rotator disposed on an optical path of the first laser beam emitted from the first semiconductor laser device and configured to rotate the polarization direction of the first laser beam by 90 degrees such that it is orthogonal to the polarization direction of the second laser beam; a polarization element (calcite, etc.) merging optical paths of the first and second laser beam of mutually orthogonal polarizations based on its birefringence effect; an optical fiber for receiving the laser beams emerging from the polarization element and letting the laser beams travel outside; and a lens for coupling the laser beams merged through the polarization element to the optical fiber. In the semiconductor laser module of this prior art, the first and second semiconductor laser devices are housed in a package to form an unit. (This technology will hereinafter be called a prior art 2.)
Further, Japanese Patent Application Laid-open No. 2000-31575 discloses a semiconductor laser module including a thermoelectric cooler; a first and a second semiconductor laser devices mounted on the thermoelectric cooler; a first and a second lenses each for collimating the first and second laser beams emitted from the first and second semiconductor laser devices; a polarization-synthesizer for synthesizing the first and second laser beams; and an optical fiber for receiving the laser beams emerging from the polarization synthesizer and letting the laser beams travel outside. Moreover, the first and second semiconductor laser devices are formed in an LD array, in which the laser diodes are arrayed at a pitch between their light-emitting centers (hereinafter referred to as inter-emission-center pitch) of 500 μm. Further, the first and second convergent lenses are formed in a lens array such as a ball lens array or a Fresnel lens array. (This technology will hereinafter be called a prior art 3.)
However, in the prior art 1, the lenses have to be aligned with respect to the respective laser beams emitted from the two semiconductor laser devices, which makes the manufacturing process complicated and requires a long time to manufacture.
In the prior art 2, the laser beams from the semiconductor laser device are directly received by a polarization rotator or a polarization element. The configuration therefore requires that a spacing between the semiconductor laser device and the lens be set to 300 to 500 μm or so in order to achieve a high coupling efficiency. It is difficult in practical point of view, however, to dispose the polarization rotator and the polarization element between the semiconductor laser device and the lens. Adopting a larger lens would create a larger space, but this approach will have a problem that a package needs to be larger in size than currently used, resulting in the semiconductor laser module being larger in size.
Further, in the prior art 3, two laser beams emitted at a wide interval (i.e. the inter-emission-center pitch of 500 μm) are respectively received by the separate lenses from each other and are made mutually parallel. The configuration has a problem that it is unsuitable for mass production since semiconductor laser devices are large in size and not obtained in large quantity from a single wafer. Narrowing a spacing between the stripes of the semiconductor laser device in order to obviate the above problem would need to accompany downsizing of the lenses, making it difficult to separate the laser beams emitted from the stripes and polarization-synthesizing or optical synthesizing of the beams that follows.
In order to solve the above problem, the applicant of the present invention has proposed a semiconductor laser module in which two laser beams emitted from two light-emitting stripes (hereinafter referred to simply as stripes) formed in a single semiconductor laser device are polarization-synthesized and received by an optical fiber. (See Japanese patent application No. 2001-383840, for example. This technology will hereinafter be called a related art.)
FIG. 22 is an explanatory diagram schematically showing a configuration of the semiconductor laser module of the related art.
As shown in FIG. 22, the semiconductor laser module M11 of the related art includes a single semiconductor laser device 2 having a first stripe 9 and a second stripe 10 formed in parallel to each other with a spacing of 100 μm or less interposed therebetween and emitting a first laser beam K1 and a second laser beam K2 from a front end face (i.e. an end face on right-hand side in FIG. 22) of the first stripe 9 and the second stripe 10 respectively; a first lens 4 positioned so that the first laser beam K1 and the second laser beam K2 are incident therealong and configured to separate the first laser beam K1 and the second laser beam K2 in the direction in which the first and second stripes 9, 10 are arrayed; a half-wave plate 6 (a polarization rotating means) configured to rotate a polarization direction of at least one of the first and second laser beam K1, K2 (i.e. the first laser beam K1 in FIG. 22) by a predetermined angle (by 90 degrees, for example); a birefringence element 7 configured to optically synthesize therealong the first laser beam K1 and the second laser beam K2; and an optical fiber 8 optically coupled to the synthesized laser beams emerging from the birefringence element 7 and letting the synthesized beams to travel outside.
In addition, a prism 5 is disposed between the first lens 4 and the half-wave plate 6 so that the first laser beam K1 and the second laser beam K2 are incident thereon and output therefrom along their respective optical axes parallel to each other. Further, a second lens 16 is disposed between the birefringence element 7 and the optical fiber 8 in order to optically couple the first and second laser beams K1, K2 polarization-synthesized by the birefringence element 7 to the optical fiber 8 which is supported by a ferrule 23.
The birefringence element 7 may be formed of a crystal such as rutile or YVO4.
The first laser beam K1 and the second laser beam K2 emitted respectively from the front end face 2a of the first stripe 9 and the second stripe 10 of the semiconductor laser device 2 travel through the first lens 4, intersect and separate until the separation between the two beams is enough, before entering the prism 5.
During propagation through the prism 5, the first laser beam K1 and the second laser beam K2 are made parallel to each other with a spacing D interposed therebetween, and are emitted from the prism 5. The first laser beam K1 then enters the half-wave plate 6, where its polarization direction is rotated by 90 degrees, and then enters a first input port 7a of the birefringence element 7, while the second laser beam K2 enters a second input port 7b of the birefringence element 7.
The first laser beam K1 incident on the first input port 7a and the second laser beam K2 incident on the second input port 7b are polarization-synthesized along the birefringence element 7, and output from an output port 7c. 
The laser beams emerging from the birefringence element 7 are then converged by the second lens 16, enter an end face of the optical fiber 8 supported by the ferrule 23, and propagate to outside.
According to the semiconductor laser module M1 of the related art, a first laser beam K1 and a second laser beam K2 polarized in identical directions are emitted from a first and a second stripes 9, 10 formed in a single semiconductor laser device 2 with an interval of 100 μm or less, and are sufficiently separated by a first lens 4. Thereafter, the first laser beam K1 experiences a rotation of its polarization direction by 90 degrees through a half-wave plate 6, when the polarization directions of the laser beams K1, K2 are orthogonal to each other. The first laser beam K1 and the second laser beam K2 are then polarization-synthesized along a birefringence element 7, and therefore, a high power laser beam of reduced DOP can be output from an optical fiber 8.
The above described semiconductor laser module Ml can therefore be utilized as a pumping light source for use in erbium-doped optical fiber amplifiers (EDFA's) demanding high output, or further in Raman amplifiers in which the low polarization dependency and the stability of amplification gain are required.
In addition, since it comprises the single semiconductor laser device 2 with the two stripes each emitting one laser beam and the single first lens 4 configured to mutually separate the laser beams K1 and K2, it takes less time to align the semiconductor laser device 2 and the first lens 4. Consequently, manufacturing time of the semiconductor laser module M1 can be shorter.
Further, since the two laser beams emitted from the single semiconductor laser device 2 travel in substantially identical directions, the optical output obtained from the optical fiber 8 can be stabilized by suppressing a warpage of a package, accommodating the semiconductor laser device 2, the first lens 4, the half-wave plate 6, the birefringence element 7, the second lens 16, etc., along only one direction (i.e. along Z-direction in FIG. 22).
Here, in the semiconductor laser module M11 of the related art, it is often required that an optical isolator 108 be interposed between the first lens 4 and the second lens 16, as shown by the broken lines in FIG. 22, allowing the first laser beam K1 and the second laser beam K2 emitted from the semiconductor laser device 2 to pass through only toward the optical fiber 8 for the purpose of preventing a reflected beam from the optical fiber 8 and thereby stabilizing the operation of the semiconductor laser device 2.
It is generally known that the optical isolator is classified into the polarization-independent isolator which permits an incident beam to pass through only in one direction irrespective of the polarization state of the incident beam, and the polarization-dependent isolator constituted by placing a Faraday rotator between input and output polarizers. The polarization-dependent isolator is further classified into one in which, referring to FIG. 23(A), a Faraday rotator 66 is interposed between a pair of polarizers 65, 65 and in which the polarization direction of the output beam is rotated by 45 degrees with respect to the input beam (hereinafter called ‘single type’), one in which, referring to FIG. 23(B), each of the two Faraday rotators 66a, 66b is interposed between two of the three polarizers 65 (hereinafter called ‘semi-double type’) and one in which, referring to FIG. 23(C), each of the two Faraday rotators 66a, 66b is interposed between two of the four polarizers 65 (hereinafter called ‘double type’). In the semi-double and double type isolators, the Faraday rotator 66a rotates the polarization of the incident beam by 45 degrees while the Faraday rotator 66b rotates the polarization of the beam emerging from the intermediate polarizer 65 by 45 degrees either in the same azimuthal direction as the Faraday rotator 66a or in the opposite to. Therefore, the polarization of the output beam is rotated either by 90 degrees or 0 degree. Additionally, in the double type isolator, a larger isolation is available due to one extra polarizer 65 added to the semi-double type.
The single type polarization dependent isolator has often been used in the semiconductor laser module because of its compactness, low loss and sufficient isolation characteristics as compared with the semi-double and double type polarization dependent isolators.
However, if the single type polarization isolator is used in the semiconductor laser module M11 of the above described related art, the laser beams will experience rotations of their polarizations by 45 degrees through the optical isolator 108, resulting in the laser beams K1, K2, having traveled through the prism 5 and the half wave plate 6, being split into ordinary rays K1n, K2n and extraordinary rays K1a, K2b upon incidence on the birefringence element 7. These rays propagate through the birefringence element 7, and as a consequence, the polarized rays depicted by dotted lines fail to reach the output port 7c, resulting in a reduced intensity of the laser beam reaching the output port 7c and inability to obtain a high output semiconductor laser module. Note that the bilateral arrows in FIG. 22 shows polarization directions of the laser beams as viewed from the optical fiber 8.